He is alone, and yet he is being watched. Neighbors awakening to their weekday obligations notice that for the man, dawn has become a kind of witching hour of stealthy activity.

For weeks, 24-year-old Jarred Harrell was observed scurrying from his shed to various areas of his backyard in the early morning. Sometimes he was dressed only in what neighbors described as short silk boxers and a gray sweatshirt.

According to a woman who lived next door, Harrell, who moved to the home in Callahan, Fla., at the start of the year, acted strangely from the start.

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"He would run from the shed back there, and just run back and forth, back and forth. He'd be out there sometimes 5:00, 5:30 in the morning just running," said the neighbor, Christie, who asked that her last name not be used.

Click HERE for a map of registered sex offenders near the school Somer Thompson attended.

"He had on like these really short... short silk... things, they kind of looked like boxers. And a gray sweatshirt, 'cause we would laugh at how he was dressed," said another neighbor who asked to be identified as Jackie.

They say he'd run for hours.

"Just like crazy running, like a kid would run when they're mad," said Christie.

As it turns out, Jarred Harrell probably had a lot on his mind. He was fast becoming a person of interest in the murder of young Somer Thompson, who vanished while walking home from school in Orange Park, Fla., on the same block where Harrell reportedly had been living at the time.

The 7-year-old girl, whose body was found in a Folkston, Ga., landfill after her October 2009 abduction, was also in Rod Buchannan's thoughts.

Buchannan, a landscaper, says he'll never forget driving down Gano Avenue -- the street where Somer went missing -- just two days after the girl's body was found. He and his wife, Lisa, suddenly spotted a familiar car parked in a driveway not far from her elementary school.

"At this point, the hair was standing up at the back of our necks. Instantly, we had that sick feeling in the pit of our stomachs. And we were nervous and I won't -- won't deny we were both scared," said Buchannan.

Jarred Harrell's Computer

Scared, because they knew the owner of the car was Jarred Harrell, someone with a dark past that they had come to fear. He was their daughter's former housemate and, from what they knew about him, the fact that he was living just three houses down from where Somer was last seen was reason for concern.

Buchannan said that less than a year earlier his son had accidentally stumbled onto Harrell's terrible secret.

"He got onto Jarred's computer one day to play a game," Buchannan said. "There was an icon on the desktop, that said, it was either 'Child Penetration' or 'Toddler Penetration.' As he turned around, Harrell came back up, and reached around and turned the computer off. And told him not to get back on the computer without his permission."

But what the Buchannans discovered later on Harrell's computer would leave no room for doubt. Harrell's mother had asked if her son could move into an apartment complex with the Buchannans' daughter and her fiancé to help him get on his feet and share expenses. Harrell did so, but the arrangement did not last long.

"Things turned up missing," said Buchannan.

Somer Thompson Suspect Reappears

One night Buchannan's daughter and her fiance say they found their iPod in Harrell's car.

"Being that he was a family friend, they give him an option," Rod Buchannan said. "Get your stuff, get out right now, middle of the night, or we call the police and put you in jail."

They say Harrell left, but in his haste left behind his computer. That would prove to be a revealing mistake. Buchannan's son had told his sister about what he had seen on Harrell's computer, and she decided to take a look for herself.

"So they decided to search the computer at that point. And that's when they found -- I mean, it was right there on the desktop," said Buchannan. "There was no effort made whatsoever to hide the material."

The siblings told their parents they found hard-core child pornography on Harrell's computer. A lot of it. And they thought he may even know some of the children.

"Stuff that our children shouldn't have seen. Should have never had to see," said Buchannan.

Buchannan said he immediately took his wife and daughter to the police and turned over the computer.

But he was unprepared to find, only two months later, Harrell's car parked at the house on Gano Avenue. The house belonged to Harrell's mother. It was in a school zone, near Somer's school.

"That was just, I don't know, just a gut instinct, you know, just get that sick, icky feeling," said Lisa Buchannan. Her husband recalled the chilling moment. "We got goosebumps," he said. "We looked at each other and I said, you know, we got to tell somebody."

The Buchannans discovered Harrell's car in the driveway just two days after police found Somer's body. They drove over to the investigation command center and spoke to the lead detective about their concerns.

"There must have been 25-30 police cars, not counting all the unmarked ones and the evidence technician vans, this place was a mad house," said Buchannan. "We informed him that our children had turned in a computer that contained a lot of child pornography. And that Mr. Harrell was being investigated by the sex crimes unit."

Strange Disappearance of Shed

The Buchannans claim they pointed out Harrell's car in the driveway to investigators. "As far as they knew that house was vacant. Police were not even aware that anybody lived there," said Buchannan.

But could police place Harrell in the house the day Somer disappeared? Public records show Harrell's family had moved out weeks earlier. Neighbors, however, remember Harrell was still coming and going from the house even after the family moved.

And shortly after the Buchannans' report, neighbors noticed something else that fueled suspicion. His shed suddenly disappeared from his house on Gano -- only to reappear at a different property.

But despite this possible evasive maneuvering and the child pornography allegedly found on his computer, Harrell wasn't the only suspect on the police department's radar.

After Somer Thompson disappeared, the residents of Orange Park were stunned to learn that their children had been walking through a virtual minefield of registered sex offenders each day: 162 living in just a five-mile radius of Somer's home.

Police Home In on Somer Thompson Suspect

Several neighbors had voiced concerns about an older man living just down the street, known as "Grandpa Charley," who they say often took little girls for rides on his bicycle. When police went to question him they say they discovered child pornography on his computer.

But further investigation cleared Grandpa Charley of any connection to Somer's murder. He has pleaded not guilty to child pornography charges. And within days police had turned their attention to another man who lived closer to Somer's school. His name is Richard Corpus, a 55-year-old handyman who works out of his home and spends a lot of time with kids.

"If I can't have kids, then I'll have somebody else's to play with, you know? I've got lots of kids that pass by," Corpus said.

ABC's "20/20" first met Corpus a few weeks after Somer was killed, standing at the corner near his house. He was greeting children on their way to school. Corpus had just gotten a new puppy and was carrying the animal inside his vest.

"I still will go out and greet kids and let them pet my little dog 'cause I think it brings happiness to the kids in the neighborhood," he said.

Corpus said he used to find Somer with her face pressed up against his chain link fence.

"Just so the dog will go and lick her face, you know? Somebody could have probably lured her very easily," said Corpus.

Over the next month police honed in on Corpus. He claims they seized his computers and monitored his calls.

"I can tell you this, every time the police walk in my house I start shaking," he said.

He said police even traveled to other states to question his sisters about his past. And that they gave him a polygraph test which he acknowledges he failed.

"They said that I failed one of them," Corpus said. "But if you sit in front with your glasses off staring a wall, I guess you do get dizzy after a while."

'I'm Not a Predator'

There is only one house between the vacant property where Somer was last seen and the corner where a crossing guard said he never saw Somer pass by. It is the house where Richard Corpus lives.

"I'm not a predator. But I can imagine, how this happened," Corpus said. "One, I'm going to look for some kids that are stragglers. Number two, I'm going to look for a kid that is -- innocent. And I'm going to plan it."

But while police were eyeing Corpus, he said he was on the lookout for the killer -- and he kept a hangman's noose in the garage just in case.

"This is not a threat, this is a promise," he said, displaying the noose for ABC cameras. "And, if I could, I'd hang him up from a light post and leave him there just to show the rest of the sexual predators what will happen. This is how extreme I want to catch this killer."

But for all this bizarre behavior, was Corpus merely a distraction in the pursuit of Somer's killer?

Jarred Harrell Tops List of Suspects

The holidays arrived, yet they brought no comfort or peace for one mother. Diena Thompson still did not have a face to put on the monster who killed her little girl and threw her away in the trash.

"I have to know for my own peace of mind what happened to her," said Thompson at the time. "I think that, what I have built up in my mind can't be any worse than what they're going to tell me."

Just a few blocks away, detectives were still keeping surveillance on Richard Corpus and waiting for his DNA results from the lab. But in early January police got the news: Richard Corpus' DNA was not a match.

That startling development moved Jarred Harrell to the top of the list of suspects. Investigators set out to get his DNA for laboratory testing. Then, suddenly, Harrell vanished. Investigators say he went to Meridian, Miss., and moved in with his aunt, who in Web posts proclaimed herself an "artist of the macabre" and exhibited a fascination for death, tortured bodies and mutilation.

Police tracked Harrell's new whereabouts and by February had enough information to take action. Harrell was officially arrested on 29 counts of child pornography -- but also was named a person-of-interest in the Somer Thompson case. Police then searched Harrell's various places of residence, including the house on Gano Avenue, in hopes of finding evidence to link him to Somer's murder.

Police teams worked late into the night, while two houses away, Richard Corpus -- the man who police had suspected early on -- was breathing a sigh of relief.

"To tell you quite honestly, if, if I didn't have a family, I'd probably be dead," said Corpus. "I probably would've killed myself around Christmas. Because that's how deep it got to me. Because I couldn't clear my name."

'Of Course He Could Have Been Off the Streets'

Police said on Harrell's camera they found explicit photos and a video of a 3-year-old whom Harrell knew. Harrell was charged with 26 more counts, including molestation and photographing sex acts.

Given the evidence against Harrell, some Orange Park residents wondered what took the police so long to get him off the street. They had Harrell's computer back in August -- why was he still living on Gano Avenue in October, when Somer disappeared?

Tami Loehrs is a computer forensics expert who has worked on several high-profile cases involving child porn.

"Of course he could have been off the streets," Loehrs said.

Loehrs said the fact that Harrell might have known at least one of his victims should have been an immediate red flag.

"That is probable cause in all of the cases I have seen," Loehrs said. "That was enough to go in and get a search warrant so that they could do a forensic preview."

Police issued a statement saying they followed protocol. The process took so long, they said, because authorities had to first determine that the child porn did, in fact, belong to Harrell.

Months earlier, before she knew who would be charged, Diena Thompson told "20/20" she wanted to meet her daughter's killer face-to-face.

For the rest of the Somer Thompson story, watch "20/20" Friday at 10 p.m. ET